Remember, A10 was first launched in the 70's When 4KB was the norm for memory shipped with computers of its time.
You can upgrade things, but there has to be continuity.
Also, checking out Bunyap's series on YT. Highly recommended.
As for the siderwinder, do you mean why it doesn't have a seeker in the back? Or do you mean why can't you fire it backwards? I'd be very leery about firing a heat seeking missile right at my engines!
Also the rails would all have to be modified, I imagine.
Finally, multiple ways of doing things makes sense to me. Gives you some redundancy.